What's the deal with the grades of Architectural and Art pencils?

Here's the code: "H" stands for "Hardness" and "B" stands for "Black."
With pencils, "hard" means a pencil that yields a lighter image. "Soft" pencils provide darker images. In this case, "black means soft.
From hardest to softest, these are the grades: 9H, 8H, 7H, 6H, 5H, 4H, 3H, 2H, H, HB, F, B, 2B, 3B, 4B, 5B, 6B.
HB is the equivalent of the "regular" number 2 pencil. The grade "F" is a grade between "HB" (hard and black) and "B." A noted pencil manufacturer contacted said that "F" was originally introduced for taking shorthand, because it was "hard" enough to withstand extended use without resharpening and black enough so as to ensure that the shorthand was subsequently legible."
The "E" grades are used to designate the hardness of Filmograph leads. These leads are produced for plastic film and used in technical drawing. Filmograph leads contain no graphite and are based upon polymers and carbon black. They are used in order to prevent the written or drawn line from being smudged when the drawing is being handled. E1 is the softest grade and E4 the hardest.
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